1. Field of the Invention
The invention generally relates to current sensing circuits and methods; and in particular, the present invention relates to a ratiometric current sensing circuit for accurately sensing the current flowing through a power-controlling pass device.
2. Background of the Invention
In circuits employing a power switch for power switching or power distribution functions, there is often a need to sense the current passing through the power switch. For example, current sensing is needed to monitor the load current passing through the power switch and the load coupled to the power switch. Current sensing is also needed to control and limit the load current in order to prevent damage to the load or to the power switch itself. Power switches are commonly implemented as n-channel or p-channel MOS devices. Although the current through the power switch can be sensed directly by placing a resistor in series with the power switch, this arrangement is undesirable because the resistor conducts the entire current through the power switch, resulting in a large power dissipation. Instead, a ratiometric current sensing technique is typically used for MOS power switches. In ratiometric current sensing, the current through the power switch is measured using a sense device which matches the power switch in electrical characteristics but is smaller by a known factor. The current through the sense device, which is a known ratio of the current through the power switch, is measured using a resistor connected in series with the sense device. The size of the sense device can be made small enough such that the current through the sense device is measured without undesirable power dissipation.
A conventional ratiometric current sensing circuit for use with a MOS power switch is illustrated in FIG. 1. Current sensing circuit 10 for sensing the current through a power device M.sub.Power and a load 13 includes a sense device M.sub.Sense and a resistor R.sub.Sense connected in series. Power device M.sub.Power and sense device M.sub.Sense are matching n-channel MOS transistors. Sense device M.sub.Sense is chosen to be K times smaller than power device M.sub.Power. Typically, K is in the range of 1000 or more. The gate terminals of power device M.sub.Power and sense device M.sub.Sense are connected together and the source terminals of both devices are connected together to a ground terminal (node 15). Therefore, power device M.sub.Power and sense device M.sub.Sense are driven with identical gate to source voltages. An input voltage V.sub.in from an input voltage source 12 is applied across load 13 and power device M.sub.Power. A load current flowing through load 13 is equivalent to the drain current I.sub.DS,P of power device M.sub.Power.
Resistor R.sub.Sense is connected between the drain terminal (node 14) of power device M.sub.Power and the drain terminal (node 16) of sense device M.sub.Sense and is used to measure the current flowing through the sense device M.sub.Sense. As long as the voltage across resistor R.sub.Sense is small compared to the drain-to-source voltage of M.sub.Sense, the drain-to-source voltages across power device M.sub.Power and sense device M.sub.Sense are essentially equal. Since the power device and the sense device have the same drain-to-source voltages and the same gate-to-source voltages, the drain current I.sub.DS,S of sense device M.sub.Sense is essentially I.sub.DS,P /K. A voltage drop develops across resistor R.sub.Sense which is equal to the product of the drain current I.sub.DS,S of sense device M.sub.Sense and the resistance of resistor R.sub.Sense.
The sensed current of sense device M.sub.Sense and the sensed voltage of sense resistor R.sub.Sense can be used to control circuit protection mechanisms for preventing excessive current flow in power device M.sub.Power and load 13. To that end, current sense circuit 10 further includes an error amplifier 20, a reference current source 19, and a reference resistor R.sub.Ref. Reference current source 19 provides a fixed reference current I.sub.Ref0 which flows through reference resistor R.sub.Ref and generates a reference voltage across the reference resistor. Reference resistor R.sub.Ref and sense resistor R.sub.Sense are either matching resistors having the same resistance values or resistors having ratioed resistance values. Error amplifier 20 compares the voltage across reference resistor R.sub.Ref (node 18) and the voltage across sense resistor R.sub.Sense (node 16) and provides a control signal on lead 17 to the gate terminals of sense device M.sub.Sense and power device M.sub.Power. In operation, the reference current I.sub.Ref0 is selected so as to set the current limit of power device M.sub.Power. Error amplifier 20 operates to limit the power device's current whenever the sensed voltage at sense resistor R.sub.Sense is equal to or exceeds the reference voltage generated by reference resistor R.sub.Ref. When a current limit condition is detected, error amplifier 20 regulates the gate-to-source voltages of power device M.sub.Power and sense device M.sub.Sense to limit the current through the sense device to the maximum allowable current value of I.sub.Ref0.
As mentioned above, in current sense circuit 10 of FIG. 1, as long as the voltage drop across sense resistor R.sub.Sense is negligible as compared to the voltage drop across sense device M.sub.Sense, the drain-to-source voltages across the power device M.sub.Power and the sense device M.sub.Sense are essentially equal and the current through the sense device tracks the current through the power device. The drain current I.sub.DS,P through power device M.sub.Power and load 13 is given by: EQU I.sub.DS,P &lt;=K*I.sub.DS,S *R.sub.Ref /R.sub.Sense, EQU =K *I.sub.Ref0 *R.sub.Ref /R.sub.Sense.
Through the use of a scaled-down sense device, current sensing circuit 10 operates at a low power dissipation level because the sensed current I.sub.DS,S is only a fraction of the power device's actual current. Furthermore, current sensing circuit 10 is applicable when the power device is biased either in the saturation region or in the linear (triode) region.
However, conventional current sensing circuit 10 has a significant drawback. In particular, conventional current sensing circuit 10 becomes grossly inaccurate when the power device is operated in the linear region where the drain-to-source voltage across the power device is small. In this case, the voltage drop across the sense resistor is no longer negligible and the drain voltage at the sense device does not track that of the power device. Thus, sense device M.sub.Sense grossly underestimates the power device's current.
For sense device M.sub.Sense to measure the power device current accurately, the terminal conditions of the two devices should be equal. That is, the gate-to-source voltages and the drain-to-source voltages should be the same for both devices. However, by virtue of the use of sense resistor R.sub.Sense, some voltage is dropped across the sense resistor. Consequently, the drain voltage at sense device M.sub.Sense is less than the drain voltage at power device M.sub.Power. In the case where the drain-to-source voltage across the power device is large, the voltage drop across the sense resistor is negligible and the drain-to-source voltages of the power and sense devices are essentially equal. However, when the drain-to-source voltage across power device M.sub.Power is small, the voltage drop across resistor R.sub.Sense is large compared with the drain-to-source voltage of power device M.sub.Power such that the drain voltage of the sense device is significantly less than the drain voltage of the power device. The disparity in the drain voltages results in a disparity in the drain current of the two devices such that the sense device grossly underestimates the current flow in the power device.
FIGS. 10a-c are graphs of the current and voltage characteristics obtained by simulation of the conventional current sensing circuit 20 in FIG. 13. Current sensing circuit 20 is constructed in the same manner as conventional current sensing circuit 10 with the only exception that the load, including load resistor R.sub.load having a resistance value of 2 ohms and load voltage source vLoad, is coupled to the source terminal of the power device M.sub.out. FIGS. 10a-c illustrate the characteristics of current sensing circuit 20 in response to a linearly ramped load current and to a short-circuit condition at the load. In FIGS. 10a-c, current sense circuit 20 is operated at an input voltage V.sub.in of 3.3 volts. Curve 178 of FIG. 10a illustrates the behavior of the load current through load resistor R.sub.load. Curve 174 of FIG. 10b illustrates the gate voltage V.sub.Gate as applied to both the sense device and the power device. Curves 170 and 172 of FIG. 10c illustrate the voltage at reference resistor R.sub.Ref (V.sub.Ref) and the voltage at sense resistor R.sub.Sense (V.sub.Sense), respectively, with reference to the input voltage V.sub.in. That is, curve 170 is actually V.sub.in -V.sub.Ref and Curve 172 is V.sub.in -V.sub.Sense. Here, reference current source iRef sets the current limit of power device M.sub.out to be 250 mA and sets the reference voltage V.sub.Ref to 50 mV.
From a time zero to a time 0.75 ms, the load current increases linearly. The gate voltage (curve 174 of FIG. 10b) increases to a maximum value of 8 volts to allow the power device M.sub.out to carry the necessary load current. Meanwhile, the sensed voltage V.sub.Sense slowly increases until the sense voltage V.sub.Sense reaches the reference voltage V.sub.Ref (50 mV) at a time of 0.5 ms, indicating that the current limit condition is reached. Current sense circuit 20 limits the load current to a value of approximately 609 mA (curve portion 178a of FIG. 10a), instead of the intended 250 mA current limit. The excessive current limit value under the ramped current condition is caused by sensing inaccuracy when the power device is biased in the linear region. For instance, at about 0.5 ms, the load current is slowly ramped up to about 600 mA. The voltage V.sub.out at the source terminal of power device M.sub.out is the voltage across load resistor R.sub.load and the load voltage source vLoad which is equal to 1.2 volts plus 2.0 volts. Thus, voltage V.sub.out is 3.2 volts. The drain-to-source voltage V.sub.DS across power device Mout is only 100 mV (3.3 volts of V.sub.in minus 3.2 volts of V.sub.out) and power device Mout is biased in the linear region. In this regime, the 50 mV voltage drop across sense resistor R.sub.Sense (denoted R1 in FIG. 13) is significant in comparison with the V.sub.DS of the power device (100 mV). The drain-to-source voltage of sense device M.sub.Sense is reduced to only 50 mV and does not approximate the drain-to-source voltage of the power device. The drain-to-source voltage disparity causes sense device M.sub.Sense to grossly underestimate the power device's current and current sensing circuit 20 does not limit the load current until the load current reaches 609 mA, far exceeding the 250 mA intended current limit.
However, when a short circuit load is applied (at time 0.75 ms), almost the entire input voltage V.sub.in of 3.3 volts is applied across power device M.sub.out and sense device M.sub.Sense and both devices are in saturation. Specifically, voltage Vout is only the voltage drop across the load resistor which is 0.52 volts (260 mA*2 .OMEGA.). Thus, the drain-to-source voltage across power device M.sub.out is 2.78 volts. The sensed voltage V.sub.Sense, being 50 mV (curve 172), is only a small fraction (1.7%) of the drain-to-source voltage of the power device. Therefore, under the short-circuit load condition, the disparity between the drain-to-source voltages of the power device and the sense device is small and sense device M.sub.Sense can accurately sense the power device's current. Current sense circuit 20 thus limits the current of the power device by lowering the gate voltage (curve 174) to about 1.5 volts. The load current is regulated down to 260 mA (curve portion 178b of FIG. 10a), closely approximating the intended 250 mA current limit. As can be observed in FIG. 10a, the value of the current limit under the ramped current condition is significantly higher than and the current limit under the short-circuit condition. The great disparity in the current limit values (a 135% discrepancy) is an indication of the sensing inaccuracy of the conventional current sensing circuit when the power device is biased in the linear region.
One prior art technique to improve the accuracy of the convention current sensing circuit is illustrated in FIG. 2. In current sensing circuit 30, a bipolar comparator, made up of pnp bipolar transistors 41 and 42, is used to keep the voltage drop across the sense resistor R.sub.Sense small. However, current sensing circuit 30 is only able to limit the voltage drop across R.sub.Sense to about 10 mV and the result is still unsatisfactory since the values of the current limits between a ramped load current and a short circuit condition still vary by over 60 percent.
Therefore, it is desirable to provide a ratiometric current sensing circuit which can accurately sense the current through a power device for all values of drain-to-source voltages at the power device. In particular, it is desirable to provide a ratiometric current sensing circuit which can sense the current through a power device accurately even when the power device is biased in the linear region.